Several known predictors of hard disk electromechanical failure include reallocated sectors, reallocated event counts, and current pending sector counts. Medium errors have not yet been studied as a predictor of disk failure rates.
When a physical problem is encountered when trying to read data from a hard disk, and multiple read attempts fail, the hard disk experiences a medium error. Medium errors can be classified as a “real medium error” or a “head failure”. A real medium failure indicates a marginal disk platter or a loss of proper magnetic properties. A head failure occurs when the read/write header has deteriorated. Conditions that may cause such an error are external conditions (e.g. dust) physically harming the disk head, imprecision in the physical write location, or improper alignment. A sudden power failure may also cause a medium error, but this is typically limited to one sector in most cases. Most medium errors are head failures or a defect on the magnetic medium.
Although reallocated sectors have been studied as predictors of disk failure, not all medium errors result in reallocated sectors. Thus, using reallocation of sectors as a predictor of disk failures is an incomplete method of predicting disk failures.